The missus was thinking about it today, and she reckons given the reaction time, she may have been doing more like 60km/h (40mph) - she's not really sure; but she wasn't speeding and wasn't tailgaiting - it's just the woman in front braked to a stop at the last minute, and very unexpectedly, which gave little time for her to react, though she was already slowing anyway, as the next set of lights was already turning red and she was pretty much aiming to stop at them.